Bring GBn2 into the simtk.openmm.app namespace. Also remove some redundant
references.
Apparently 1/u.nanometers is a quantity rather than a unit, so the command:

implicitSolventKappa = implicitSolventKappa.value_in_unit(1/units.nanometers)

will always fail. The only solution I've found is to take the unit of the
quantity. i.e.:

implicitSolventKappa = implicitSolventKappa.value_in_unit((1/units.nanometers).unit)
Merge pull request #260 from swails/master

Add GBn2 to the simtk.openmm.app namespace and correct unit handling of implicitSolventKappa